In the usual prior art optical recorders, a pulse modulated coherent laser beam is recorded on the radiation-sensitive stratum of a rotating disk, cylinder, or other appropriate medium. In the more common video disk version, the modulated laser beam records distinct pulses in tracks or grooves on the disk of micrometer width or less. The tracks or grooves form a continuous spiral path, or a series of concentric circular paths. The paths may be formed by the beam during the recording process, or the beam may be made to track pre-existing paths formed in the medium prior to the recording process.
A typical digital optical record produced by the recording process described above may consist of a series of optically transformed microspots each of which may be considered to represent a binary "1," and which are separated by other areas which may be considered to represent a binary "0." Each of the binary "1's" may have the same duration, or period; and each of the binary "0's" may a period equal to the periods of the binary "1's," or to some multiple thereof.
Because of the minute size of the recorded spots on the prior art optical recordings, errors are inevitable due to inconsistencies in the materials, as well as defects in the components and in the recording process itself. The resulting errors are usually referred to as "dropouts."
The detection of dropouts in the prior art optical recording systems has usually been based on techniques learned from the prior magnetic recording systems. In the prior magnetic recording systems, the success or failure of a first magnetic recording head to record a binary "1" is detected by a second monitoring head which is physically displaced along the recording medium from the first head. This particular monitoring technique involves a delayed analysis of the recording process, and it requires some form of re-recording in order to achieve overall substantially error-free recording. Examples of such prior art optical recording systems may be found, for example, in U.S. Pat. Nos. 4,308,612, 4,464,805 and 4,669,072. The optical recording systems disclosed in each of the patents include means for monitoring the signal being recorded. That is, each of the optical recording systems described in the patents uses a monitoring technique similar to that used in magnetic recording, and which causes the flawed segments of the recorded signals to be re-recorded in a different space on the recording medium.
U.S. Pat. No. 4,308,612, for example, makes reference to apparatus for magnetically recording digital information on a magnetic disk which includes error checking means. It is pointed out in the patent that, in optical disk systems for optically recording digital information in a sequence of binary "1's" and "0's" on a recording disk, checking operations similar to those used in magnetic recording are necessary in order to reduce dropouts.
U.S. Pat. No. 4,308,612 describes optical recording apparatus capable of performing a read-while-write operation in which a reflected beam from the recording medium is detected during recording, and the data being recorded are checked on the basis of the reflected beam. When it is found that the data being recorded are erroneous, the same data are correctly recorded on the next sector of the disk the number of times necessary to achieve acceptable results. The prior art system of U.S. Pat. No. 4,051,329 advances the art of optical recording a step further by indirectly measuring the energy of each pulse being recorded, and storing the energy in a sample-and-hold circuit for improving the recording of the next or following pulses.
The system of U.S. Pat. No. 4,562,567 advances the art of optical recording yet another step by employing two levels of intensity for the recording beam, with the second lower intensity level being initiated by a quantum drop in the monitored level of the beam reflection resulting from pit formation in the surface of the recording medium, as compared with a selected preset level.
In all of the prior art optical recording systems described above, indirect delayed and/or partial means are resorted to in order to achieve error compensation in the recording process. In the practice of the present invention, however, the recordable stratum of the optical recording medium itself actively interacts with the signal being recorded on a feedback power-demand basis to provide sufficient error reduction so that perfect recording may be achievable on one pass. By the system of the present invention, it is possible to achieve significant dropout error reduction in real time, for errors due to variations in the sensitivities of the recordable strata of different recording media, and due to speed variations of the recording media with respect to the recording beams, as well as due to occlusions and other anomalies in the media.